Punta Salinas radar blanking test

May 2006

Intro

The punta salinas radar currently blanks its
radar beam
when it points +/- 45 degrees from the observatory direction. The
radar
had undergone extensive maintenance during the first part of 2006.
The
people at punta salinas requested that we recheck the blanking. They
wanted
to know if it was working properly and if it was possible to narrow
the
blanking sector north of the observatory (the part that goes out to
sea).

The blanking tests.

On 09may06 we did a coordinated test with punta
salinas.
We took data at various blanking interval settings to see how the
received
signal varied at AO. The table below shows the test number, blanking
intervals
used, start time for test, and the rms of the measured power from
the a/d
converter.

TestNo.

Blanking Region

Start Time(Ast)

Min BeteewnTests

A/D sigma levels1382nb,1400nb,1400wb

Notes

1

(+45,-45)

13:29:16

0

6.5,5.6,1.4

Current blanking

2

(+30,-45)

13:40:17

11

6.4,5.5,1.2

3

(+30,-45)

13:45:18

5

1.1,5.6,1.1

Add 10db pad in front of square law detector.

4

(+20,-45)

14:06:44

22

12936.5,5.7,1.1

take out 10db pad.

5

(+10,-45)

14:19:09

12

6.2,5.5,1.2

6

(+10,-45)

14:22:57

4

1.1,5.5,1.2

Add 10db pad in front of square law detector

7

(+40,-45)

14:30:10

7

6.2,5.5,1.2

8

(+90,-90)

14:37:46

8

6.1,5.4,2.3

check for back lobes.

For the blanking intervals, +=North of Ao, -=South of AO.

The minutes between tests column shows that there was not the
same time
interval between each test. This is why some of the airplane
echoes jumped
differently between some adjacent tests.

The A/D levels on the noise (col 4) remained constant except for
the 2
tests (test3 and test6) where we inserted a 10 db bad in the
1382nb signal
path before the square law detector.

Data taking setup:

The punta salinas radar transmitted on channel 18. This uses the
frequency
bands:1366.615-1367.765 and 1381.515-1382.765. We chose channel
18 to be
away from the other radars.

During these tests the aerostat radar was off. The FAA radar
(1330,1350)
and the remy radar (1270,1290) were transmitting.

The az, gregorian dome was parked at az=270, za=18
degrees.

The lband wide receiver was used in linear polarization mode.

Three separate bands of data were recorded. Each was square law
detected
with a 20 usec time constant and then sampled at 10 usecs using
a 12 bit
a/d converter.

A 5 Mhz band at 1382 Mhz. This measured the power from the
1382 band of
the punta salinas radar. For tests 3 and 6 an extra 10 db pad
was placed
in front of the square law detector so the a/d would not clip.

A 5 Mhz band at 1400 Mhz. This measured the compression of the
system caused
by radars.

A 500 Mhz band centered at 1400 Mhz. This allowed us to
identify different
radars by their ipps.

About 200 seconds of data was taken during each test. This is
about 16
12 second rotations of the radar.

Making the plots:

For each test the data was processed using:

The start of the southern blanking period was used to register
the plot.
The data was split into separate rotation periods. It looks like
the rotation
period is actually 12.019 seconds. This gave 1.2019e6 samples
per radar
rotation.

The rotation phase of the radar was computed. I defined 0 phase
to be the
time when the radar pointed at the observatory. The observatory
is at az=257
degrees when measured from the radar itself (with a clockwise
from above
rotation).

The 1382 Mhz total
power
versus rotation angle as we change the blanking:

a peak hold over 100 samples (1 millisecond) was done for each
test. This
was to decrease the data so it could be plotted. The pulse
lengths for
the radar are 51.2 and 409 useconds.

The data was plotted as total power versus rotation phase (ao
based). Each
rotation was offset from the previous for display purposes. The
vertical
access is linear in power. In most cases the A/D converted
saturated during
part of the rotation phase.

Each line of the plot is a separate test. At each phase
of
the radar a peak hold was taken over the N rotations for this
test. Some
of the things you can see in this plot are:

The AO blanking is easily seen. The start of the blanking
remains constant
since most tests started with blanking at -45 Degrees (south)
of AO. The
right edge of the blanking changes depending on the blanking
interval of
the test.

There is a second blanking region centered at 210 degrees from
AO zero.
This blanking is at the airport. The larger signal about the
airport blanking
region is coming from radar reflections from planes above the
airport getting
back to AO.

There are two large peaks that drift. One to positive phase
and one to
negative phase. The apparent motion of these peaks between
tests varies
because there was a different time interval between the start
of each test.
Both of these birdies saturated the digitizer. The birdies
were:

Starting at phase=105 degrees and moving to higher phase. It
disappears
in the fifth test. It was present until an azimuth of 180
degrees.

Starting at phase=55 and moving toward lower phase. It
disappears on the
7th test. It was last seen at a phase of 10 degrees.

Online we thought that these large peaks were back lobes of
the radar.
After looking at the summary plot it is clear that these are
reflections
from airplanes. The birdie with increasing phase was moving
west to east.
It probably disappeared when it landed at either isla grande
or the isla
verde airport. The second birdie was a plane
moving east to
west. It disappeared due north of the observatory (0
degrees) when we went
back to 40 degree blanking. The power in these peaks is much
larger than
the radar far out sidelobes. See test 3: +30 blanking with
10db pad. These
two peaks stick well above any non blanked sidelobes.

The phase less than 0 (south of AO) shows sidelobes at (-50, -
65, and
-80). The negative blanking was never less than -45 degrees.

The positive phase (north of AO) shows sidelobes at 40, 62,
and 80 degrees.
Some of the closer in sidelobes are swamped by the reflection
from the
plane.

This is the same blanking as test2. We added a 10 db pad
before
the square law detector. The peaks at 42 and 130 are airplanes.
The 15th
rotation shows a large increase in the power from the airplane at
130 degrees.
It must have turned so that we were getting a spectral reflection
between
the radar and us.

The airplane at small angle has moved inside the
blanking region
so we don't see it. Most of the close in sidelobes must be now
coming from
the beam rather than the airplane.

The 2nd airplane is at 165 to 175 degrees. Its strength drops
dramatically
after the 15th rotation. It goes from very strong to barely
noticeable
in 12 seconds. The plane must have dropped low enough to not
reflection
much back in our direction. It looks like it was still flying
on rotation
22 as it neared the isla verde airport.

This is 5 degrees less blanking than we normally use.
There
are no airplane reflections in this data set (other than the
airport).
The sidelobe at 40 degrees is being cut down pretty well except
for rotation
10.

This has the radar completely blanked the half of its
rotation
period that points at the observatory. The only thing we see are
the planes
flying close to the san juan airport. The one spike on the second
trace
around -60 degrees is probably some other rfi.

The 5 Mhz band about 1400 Mhz was plotted in red. When it went
negative,
the system compressed.

The radar band was over plotted in black: either the 5 Mhz band
at 1382
(punta salinas) or the 500 Mhz band about 1400 (all radars).
This let you
line when the compression occurred with the various radars.

Fig1 : Test 5 punta salinas blanking of +10 deg north.
The
9 12 second rotations were reduced to 1 rotation. For the radar
bands
a peak hold over the 9 rotations was used. For the red
compression band
a min hold was used. The data was then reduced to 1 millisecond
resolution
using the same peak/min holds.

Top: The black line is the 1382 Mhz band. The peak at az=11 is
from the
sidelobe of the punta salinas radar. The red line is the 5 Mhz
wide 1400
Mhz band (it was scaled up for display). There is no dip in
the red line
at az=11. You do see a dip at az=0 degrees.

Center. The 500 Mhz band centered at 1400 Mhz. This contains
all of the
radars. The red dip at az=0 is being caused by a radar at
az=0.

Bottom. Plotting the spacing between the ipps of the radar at
az=0 shows
that the ipp is about 2750 useconds. This is the ipp of the
remy radar.
Looks like the remy radar was pointing at AO the same time
that the punta
salinas radar was.

Fig 2: Test 2,3 compression from reflections from planes. These
plots
are single 12 second rotations at 10 usec resolution. The red
lines
are the 5 Mhz band at 1400 Mhz. The blank lines are the 1382 Mhz
band (punta
salinas).

Top: Test 2 rotation 14, strongest plane reflection at az=45
degrees. The
red line does not show any compression during this reflection.

Center: Test 3 rotation 15, the strongest reflection
from the plane
at az=130. There is a small compression at az=131.76

Bottom: blowup showing the compression (blue dotted line). The
width of
the pulse causing the compression is about .015 degrees. This
is about
500 useconds for a 12 second rotation period. The compressing
pulse is
one of the wide (410 usec pulses) of the punta salinas radar.

Fig 3: Test 3 compression by other radars.

Top: test 3 rotation 15. The black line is the 500 Mhz band
centered at
1400 Mhz. You can see 3 spikes in this rotation: az=0 remy
radar, az=60
faa radar, az=130 plane reflection from punta salinas radar.

Center: blowup around remy radar at az=0. You can see the red
lines going
negative on many of the remy radar ipps.

Bottom: blowup around faa radar at az=60. Very few of the red
lines go
negative on the faa radar pulses.

We know that there is a strong azimuth dependence
on
the amount a signal from the horizon gets into the receivers at AO
(see
az
dependence
of faa radar using alfa). Az 270, za=18 is not the worst
spot for the faa radar. That is probably why we did not see any
compression.
The same may hold true for the punta salinas radar.

Summary:

The radar has a rotation period of 12.019 seconds.

The punta salinas echoes from planes are many times stronger
than the sidelobes
of the radar (at least the sidelobes beyond 10 degrees). For
test3 rotation
15, the power from the plane at az=130 degrees was 15 db higher
than the
sidelobe at 60 degrees.

The strength of the echoes from planes can change a lot (5-10db)
in a 12
second rotation period. This is probably do to the
geometry between
the radar, plane, and AO changing.

The sidelobes that we were able to see (no plane reflections
present )
occurred at:

North of AO: 10 degrees, 40 degrees, and 60 degrees.

South of AO: -50, -65, and -80 degrees.

Changing the positive blanking interval changes which side lobes
we see:

+45 degrees is the current blanking. The +40 degree sidelobe
is not seen.

+10 degree blanking. A large sidelobe around az=12 degrees
appears. It
is much larger than the 40 or 60 degree sidelobe.

The strongest signals that we receive from the punta salinas
radar are
echoes from planes (not the direct path from the radar. This is
true down
to +10 degree blanking.

We see no compression from the punta salinas radar from its
sidelobes down
to +10deg blanking. This statement is true for the gregorian
position of
az=270, za=18. There are other az,za's of the telescope that
would have
stronger radar returns.

We do see small compression from the 410 usec pulse of the punta
salinas
radar when it reflects off of a plane (test 3 rotation 15,
az=130).

The system was also being compressed by the remy radar. We saw
no compression
from the faa radar for this az,za.

It is important to remember that these results will vary as we
move the
telescope az,za.